travelling a distance burns the same calories in a person irrespective of your speed.
eg. if *I* travel 10k in 45minutes, or walk it in 90, same calories.
At the lower speed a higher % of the energy is sourced as fat, and at the higher speed a higher % is glycogen.
Now, what "they" will have you believe, is that if you want to lose fat, you must go slow/steady. This is nonsense I'm afraid.
Since at 8mph you are burning double the energy per minute you are over 4mph it's irrelevant that the % fat is smaller, the *ACTUAL* fat volume is great, albeit a smaller % of the total. So for example 90% of 100calories as fat is smaller than 50% of 200calories at 8mph.
If you go out and maximise your hour training by travelling fast, then you burn more calories, and fat. However, if you then cannot train the next day even gently, then you can see how your weekly burn can be affected by time-on-feet. 10mph is all well and good, but not if you need 3 days to recover from it.
I've got *loads* more detail than that if anyone is really interested, but that's the gist.
Travel at your highest speed you can, but maintain the ability to train tomorrow or you may as well go a little slower (this is for weight loss).
Oh, and remember - when you run HARD, the affect of the exercise lingers in the body later as it repairs you once you have stopped, so the calorific burn continues deep into your rest period, and furthermore, regular hard exercise increases the metabolic rate by an everage of 7%, so just sitting at your desk the day after hard exercise means you are burning around 107% of what you would had you not exercised. This effect wears off after around 3 days.